Trailing Shields and Trailing Shield Packages:Trailingshields.com

Why use Trailing Shields

I have worked in the welding industry for almost 30 years. During that time I held a variety of positions from welder, welding technician, welding engineer, sales engineer, consultant. 20 of those years was/is dedicated to the fluid mixing industry where exotic alloys like titanium and zirconium are used daily. This is where I gained my knowledge and experience welding outside the bubble.

Through many years of trial and error I have developed a user friendly trailing shield. Please take some time and view the welds below that were accomplished by using trailing shields. As you can see, the weld quality is excellent. Click on the trailing shield thumbnails to enlarge the pictures.

Important differences between titanium and steel or nickel-base alloys need to be recognized. These are:

►Titanium’s lower density
►Titanium’s lower modulus of elasticity
►Titanium’s higher melting point
►Titanium’s lower ductility
►Titanium’s propensity to gall
►Titanium’s sensitivity toward contamination during welding

Titanium and its alloys are most often welded with the gas tungsten-arc (GTA or TIG) and gas metal-arc (GMA or MIG) welding processes. Resistance, plasma arc, electron beam and friction welding are also used on titanium to a limited extent.

Protection needs to be provided to titanium weldments on cooling down to about 800°F (427°C) as well as to the molten weld puddle in order to prevent contamination by air. During GTA and GMA welding, argon or helium shielding gases of welding grade with dewpoint of -50°F (-46°C) or lower are used to provide the necessary protection. Separate gas supplies are needed for:

►Primary shielding of the molten weld puddle.
►Secondary shielding of cooling weld deposit and associated heat affected zones.
►Backup shielding of the backside of weld and associated heat affected zones.

Primary shielding of the molten weld puddle is provided by proper selection of the welding torch. Standard water-cooled welding torches equipped with large (3/4 or 1-inch) ceramic cups and gas lenses, are suitable for titanium. The large cup is necessary to provide adequate shielding for the entire molten weld puddle. The gas lens provides uniform, nonturbulent inert gas flow.

Argon is generally used in preference to helium for primary shielding at the torch because of better arc stability characteristics. Argon-helium mixtures can be used if higher voltage, hotter arc and greater penetration are desired. Manufacturer’s recommended gas flow rates to the torch should be used. Flow rates in the vicinity of 20 cfh have proven satisfactory in practice. Excess flow to the torch may cause turbulence and loss of shielding. The effectiveness of primary shielding should be evaluated prior to production welding. An arc can be struck on a scrap piece of titanium with the torch held still and with shielding gas only on the torch. The shielding gas should be continued after a molten puddle forms and the arc is extinguished, until the weld cools. Uncontaminated, i.e., properly shielded, welds will be bright and silvery in appearance.

Secondary shielding is most commonly provided by trailing shields. The function of the trailing shield is to protect the solidified titanium weld metal and associated heat-affected zones until temperature reaches 800°F (427°C) or lower. Trailing shields are generally custom-made to fit a particular torch and a particular welding operation. Design of the trailing shield should be compact and allow for uniform distribution of inert gas within the device. The possible need for water-cooling should also be considered, particularly for large shields. Porous bronze diffusers have provided even and non-turbulent flow of inert gas from the shield to the weld.

The prime purpose of backup devices is to provide inert gas shielding to the root side of welds and their heat-affected zones. Such devices often look much like trailing shields and may be hand-held, or clamped or taped into position. Water-cooled copper backup bars (or massive metal bars) may also be used as heat sinks to chill the welds. These bars are grooved, with the groove located directly below (or above) the weld joint. About 10 cfh of inert gas flow per linear foot of groove is required for adequate shielding.

Makeshift shielding devices are often employed very effectively with titanium welds under shop or field conditions. These include use of plastic to completely enclose the workpiece and flood it with inert gas. Likewise, aluminum or stainless steel foil “tents,” taped over welds and flooded with inert gas, are used as backup shields. When such techniques are used, it is important that all air, which will contaminate welds, be purged from the system. An inert gas purge equal to ten times the volume of the air removed is a good rule-of-thumb for irregular spaces. A moderate rate of inert gas should be maintained until the weld is completed.

Argon is generally selected in preference to helium for use in trailing shields and backup devices, primarily because of cost but also because it is more dense. Helium, with its lower density, is sometimes used for trailing or backup shielding when the weld is above the device. It is important that separate flow controls are available for primary, secondary and backup shielding devices. Timer controlled pre-purge and post-purge of torch shielding, and solenoid valves with manual switches interlocked with the welding current for secondary and backup shielding are also useful.

Bend tests evaluate ductility. For this reason, the bend test made on preproduction trial welds or on extensions of production welds made for that purpose, provides a good evaluation of weld quality. A bend sample in which the weld is positioned perpendicular to the bend axis assures uniform straining of weld metal and heat-affected zones, thereby giving more meaningful results. Table 1 lists weld bend radii for various titanium alloys.

Table 1. Bend radii for titanium welds

ASTM Grade	Weld Bend Radius
1	2T
2	3T
3	4T
4	4T
5	10T
7	3T
16	3T
11	2T
17	2T
12	5T

Good quality welds should be capable of being bent to the indicated radii without cracking. Problems with titanium welds are generally a result of contamination due to inadequate shielding. The color of welds can be used as an indicator of shielding effectiveness and, indirectly, weld quality. Thus, any indication of the quality level of a single pass titanium weld is readily apparent to the welder and any inspector. Weld colors reflect the degree to which the weld was exposed to oxygen (air) at elevated temperature. A bright silvery metallic luster generally can be taken as an indication of a good weld, provided the weld joint was clean and good techniques were followed.

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Why use Trailing Shields